In the production of aggregate materials, such as sand, gravel, crushed rock, scalpings and iron and other mineral ores, the desired product material will often be contaminated with clay or other soils, organic material such as grass, roots, and small wood or tree branch fragments, and man-made waste materials, such as light plastics. In order to prepare such aggregate product material for market, it is necessary to process the product material so that it is free from such contaminants.
For heavily contaminated aggregate materials this is often achieved in what is known as a “logwasher”. A logwasher typically comprises a trough mounted at an inclined angle relative to horizontal to which water is added, the feed material being delivered into a lower end of the trough. A pair of parallel shafts or logs are rotatably mounted within the trough and are driven to rotate in opposite directions. Each shaft has a plurality of paddles or blades mounted thereon, usually mounted to the shaft at an angle, the paddles on adjacent shafts being staggered so that mud balls and other clumps of material are broken down by attrition of the material between the blades of the adjacent shafts. The blades are angled so that they carry the product material towards the raised end of the trough, where the separated and washed product material is discharged, typically onto a grading and/or dewatering screen.
Contaminants separated from the product material, typically comprising grass, roots, twigs and light plastics, (known as trash) typically float on top of the water in the trough. It is known to provide a weir in the lower end wall of the trough, such weir defining the maximum water level within the trough, while excess water and floating trash may pass over the weir to exit the trough.
It is necessary to dewater such trash before it can be collected and disposed of. Therefore is known to provide a dewatering screen beneath the trough onto which the trash can be collected and dewatered after leaving the trough entrained in a flow of water via the weir. However, the arrangement of such dewatering screen beneath the lower end of the trough of the log washer increases the overall height of the apparatus and requires additional conveying arrangements for conveying the dewatered trash from the deck of the dewatering screen onto a suitable stockpile. Fine material and water may be collected in a sump of the dewatering screen for further processing and/or disposal.